IL-12 is a heterodimeric cytokine which provides a key link between the innate and acquired immune systems (Trinchieri, 1995, Ann. Rev. Immunol. 13:251). The functional heterodimer comprising IL-12, named p70, is composed of the covalently linked, glycosylated products of two separate genes: a heavy chain subunit named p40, whose expression is tightly regulated in cells which secrete the intact heterodimer, and a light chain subunit named p35, which is constitutively expressed at low levels in most cell types that have been studied.
Production of IL-12 in animals is stimulated in response to the presence of bacteria (e.g., Staphylococcus aureus), bacterial products (e.g., endotoxin/lipopolysaccharide (LPS)), protozoa (e.g., Toxoplasma gondii), viruses (e.g., mouse cytomegalovirus) and mimics of viral double stranded (ds) RNA replicative intermediates (e.g., polyinosinic acid:polycytidylic acid) (Trinchieri, et al., 1995, J. Virol. 69:1955); Manetti et al., 1995, Eur. J. Immunol. 25:2656).
IL-12 is critical for the development of cell-mediated immunity (CMI) in a mammal in that it is a potent inducer of IFN-.gamma. production in T cells and NK cells, it is co-mitogenic for T cells and NK cells, it is required for the development of Th1 responses, it is necessary for DTH responses, and it is an enhancer of NK cell cytotoxicity (Trinchieri, et al., 1994, Blood, 84: 4008; Muller et al., 1995, J. Immunol. 155:4661). Monocytes are the principal producers of IL-12 in peripheral blood mononuclear cells (PBMC), and monocyte/macrophages are believed to be the principal IL-12 producing cells in vivo (Trinchieri et al., 1992, J. Exp. Med. 176:1387; Gazzinelli et al., 1993, Proc. Natl. Acad. Sci. U.S.A. 90:6115).
In the case of a number of viruses which perturb the CMI, monocyte-macrophages are prime target cells during natural infection with these viruses. Such viruses include measles virus (Esolen et al., 1993, J. Infect Dis. 169:47; Moench et al., 1988, J. Infect. Dis. 158:433). Infection with measles virus is accompanied by marked and prolonged abnormalities in the CMI response which are believed to contribute to the increased susceptibility to secondary infections in the mammal that account for most of the morbidity and mortality of the disease. Sensitization and expression of delayed-type hypersensitivity (DTH) responses are inhibited in vivo in an animal for several weeks following acute measles infection (von Pirquet, 1908, Deutsch. Med. Wochenschr. 30:1297; Tarnashiro et al., 1987, Pediatr. Infect. Dis. J. 6:451). In vitro, lymphoproliferative responses to antigen, recall antigen and mitogen are suppressed, and NK cell activity is markedly decreased (Whittle et al., 1978, J. Clin. Invest. 62:678; Arnebom et al., 1983, Infect. Immun. 39:29; Hirsch et al., 1984, Clin. Immunol. Immunopathol. 31:1; Griffin et al., 1990, Clin. Exp. Immunol. 81:218). There is additional in vivo and in vitro evidence of a type 2 polarization in cytokine responses during and following measles virus infection which is evidenced by the elevated production of IL-4 and decreased production of IL-2 and IFN-.gamma. (Griffin et al., 1993, J. Infect. Dis.168:275). Immunization with live attenuated measles virus vaccine produces similar abnormalities in CMI (Hirsch et al., 1981, Clin. Immunol. Immunopathol. 21:341; Zweiman et al., 1971, Int. Arch. All. Appl. Immunol.40:834; Ward et al., 1993, Clin. Immunol. Immunopathol.67:171).
Vaccination of infants with a high-titer measles virus vaccine is associated with an increase in mortality suggesting that, similar to the situation during infection with wild-type infection, the alterations in CMI induced by infection with the vaccine strain may be of significant clinical importance (Garenne et al., 1991, Lancet 338:903; Aaby et al., 1993, J. Pediatr.122:904; Holt et al., 1993, J. Infect. Dis.168:1087). The mechanisms responsible for the marked suppression of CMI associated with measles virus infection remain obscure.
The receptor for measles virus is CD46. This protein is also known as membrane cofactor protein, a cell-surface member of the Regulators of Complement Activation (RCA) gene cluster (Naniche et al., 1993, J. Virol.67:6025; Dorig et al., 1993, Cell 75:295 1993). RCA family members control fluid phase and membrane amplification of complement activation at the pivotal C3 step and are related genetically, being tightly clustered on chromosome 1. RCA members are also related structurally, being composed of repeating motifs known as short consensus repeats (SCRs) consisting of about 60 amino acids with 4 invariant cysteines and 10-18 other highly conserved amino acids. These members are related functionally as well, in that they bind the complement activation products C3b and C4b (Naniche et al., 1993, J. Virol.67:6025; Dorig et al., 1993, Cell 75:295). Members of the RCA family of proteins include, among others, CD46, CD55, CD21, CD35 and CD59.
CD46 contains 4 SCRs. The binding site for measles virus hemagglutinin on CD46 involves SCRs 1 and 2. C3b binds to SCR's 3 and 4 and C4b binds to SCR's 2, 3 and 4 (Gerlier et al., 1994, J. Gen. Virol.75:2163; Nussbaum et al., 1995, J. Virol. 69:3341; Manchester et al., 1995, Proc. Natl. Acad. Sci. USA 92:2302; Adams et al., 1991, J. Immunol. 147:3005). The role of CD46 in complement activation and in complement-mediated disorders is discussed in WO91/18097. Further, anti-CD46 antibodies have been used to block the action of complement regulatory proteins in order to prevent the degradation of complement (Azuma et al., 1995, Scandinavian J. Immunol. 42:202-208). Since the Azuma et al. study involved the use of anti-CD46 antibodies to mediate complement lysis of human tumor cells which were present in a mouse, it was not possible to examine the overall effect of administration of anti-CD46 antibodies on the immune system of the mouse because mouse cells do not react with these antibodies and to date, no mouse homolog of human CD46 has been identified. The use of anti-CD46 monoclonal antibodies to block measles virus infection has been reported (Seya et al., 1995, Immunol. 84:619-625). However, the mechanism by which this inhibition was effected has not been disclosed.
Complement receptor 3 (CR3) is a member of the .beta.2 integrins family of heterodimeric cell surface proteins, is composed of two polypeptide chains, .alpha. and .beta., and is a complement receptor present on monocytes/macrophages, neutrophils, and NK cells. CR3 has two binding domains: a lectin domain, which mediates the binding of several bacteria, intracellular parasites, zymosan, .beta.-glucan, I-CAM, fibrinogen, CD14/LPS, and an I domain, which binds iC3b (Ross et al., 1993, Clin. Exp. Immunol. 92:181-184). Previously published reports established that administration of antibody against CR3 to animals suppressed the delayed type hypersensitivity (DTH) reaction and exacerbated infection with Listeria monocytogenes or Toxoplasma gondii (Conlan et al., 1992, J. Leuk. Biol. 52:130-132; Johnson et al., 1996, Infect Immun. 64:1998-2003).
Mononuclear cells obtained from EHV patients are markedly impaired in their ability to produce IL-12 (Chehimi et al., 1994, J. Exp. Med.179:1361), suggesting the possibility that such aberrant IL-12 production has a pathogenetic role in the immunodeficiency associated with HIV infection.
IL-12 is critical for the generation of CMI responses. CMI responses are critical for clearance of and immunity to a wide variety of pathogens (Castro et al., 1995, J. Immunol. 155:2013; Tripp et al., 1994, J. Immunol. 152:1883; Heinzel et al., 1993, J. Exp. Med. 177:1505; Sypek et al., 1993, J. Exp. Med. 177:1797; Scharton-Kersten et al., 1995, J. Exp. Med. 154:5320; Urban et al., 1996, J. Immunol.156:263; Zhou et al., 1995, J. Immunol. 155:785; Zhang et al., 1994, J. Clin. Invest.93:1733; Modlin et al., 1996, Res. Immunol. 146:526). However, CMI responses may also be pathogenic. There is clear evidence that inappropriate CMI responses accompanied by dysregulated type 1 cytokine expression and/or pathological DTH responses, are integral to the development of several immunologically-mediated disorders. A pathogenic role for IL-12 has been implicated in several of these conditions, including rheumatoid arthritis (Germann et al., 1995, Proc. Natl. Acad. Sci. USA 92:4823), multiple sclerosis (Windhagen et al., 1995, J. Exp. Med. 182:1985; Leonard et al., 1995, J. Exp. Med. 181:381; Segal et al., 1996, J. Exp. Med. 184:771), graft-versus-host disease (Via et al., 1994, J. Immunol. 153:4040; Williamson et al., 1996, J. Immunol. 157:689), diabetes mellitus (Trembleau et al., 1995, J. Exp. Med. 181:817; Adorini et al., 1996, Res. Immunol. 146:645), sarcoidosis (Moller et al., 1996, J. lmnunol. 153:4952), granulomatous colitis (Neurath et al., 1995, J. Exp. Med. 182:1281; Neurath et al., 1996, J. Exp. Med. 183:2605), systemic lupus erythematosus and Crohn's disease.
There remains a need in the art for a method for suppressing unwanted CMI responses in certain disease states. There also remains a need for the development of agents which function to suppress unwanted CMI responses in certain disease states. The present invention satisfies this need.